1. Field of the Invention
The present invention generally relates to the manufacturing of bipolar devices. It applies to the manufacturing of bipolar transistors in pure bipolar technology or in mixed bipolar-CMOS (BICMOS) technology.
2. Discussion of the Related Art
Manufacturing the base-emitter junctions of bipolar transistors raises different problems. It is in particular desirable to have self-aligned manufacturing processes to reduce the dimensions of the devices.
It is also desirable not to implant the base, which inevitably creates gaussian doping profiles. The base is ideally formed by a doped epitaxial deposition.
It is also desirable to minimize the access resistance to the base (base resistance) and to improve the frequency response of the transistors. For this purpose, it has in particular been provided to use a silicon-germanium layer as a base. However, the use of germanium, although considerably improving the access resistance of a bipolar transistor and the transit time through the base, raises implementation problems. It is difficult to bring this germanium, by an implantation, into a silicon layer, and such a layer does not withstand high temperatures (the temperature must remain stable under 900.degree. C.; a fast thermal anneal no more constraining than 1030.degree. C. may however be tolerated for 20 s).